Quickly giving morphine to wounded troops cuts in half the chance they will develop post-traumatic stress disorder, according to a provocative study that suggests a new strategy for preventing the psychological fallout of war.

Researchers at the U.S. Naval Health Research Center led the study of about 700 troops injured in Iraq from 2004 through 2006.

About 53,000 troops returning from Iraq and Afghanistan have been treated for PTSD, a disorder in which someone who has endured a traumatic event keeps re-experiencing it and the fear it caused. Patients often have trouble with work, relationships, substance abuse and physical ailments.

Researchers have been testing ways to treat it, and the new study looked at whether fast and strong pain relief can help prevent it.

It was unclear whether it was the fast pain treatment or something specific to morphine that made the difference.

But researchers theorize that simply easing pain might reduce the severity of the psychological trauma, or that prompt relief might alter the way the brain remembers the attack or injury — in essence, causing the mind to file away the episode as less traumatic.

Troops in the study initially were treated at military medical facilities in Iraq, mainly for wounds caused by roadside bombs, bullets, grenades or mortar fire. A few dozen had burns or were hurt in crashes or falls. The decision on whether to give morphine was up to the individual doctor, based on the patient’s condition.

Of the 696 troops in the study, 493 — about 70 percent — were given morphine, most within an hour of injury. Two years later, 147 of them had developed PTSD. Of the 203 not given morphine early on, 96 developed PTSD.

That worked out to a 53 percent lower risk of developing PTSD for those treated early with morphine. No other factor, such as the nature or severity of injuries, had much effect on the chances of developing PTSD, Holbrook said.

“These are provocative and thought-provoking findings that should lead scientists to investigate the underlying mechanisms” in future studies, said JoAnn Difede, a PTSD researcher at New York-Presbyterian/Weill Cornell Medical Center.